Look at the map of the modern Middle East and what jumps out are the number of ruler-straight lines that describe the borders defining Syria and its neighbors Turkey, Jordan, Iraq, Lebanon and Israel. These nation-states were all invented less than a hundred years ago, on the drawing boards of France and Britain as they gerrymandered what became a series of minority-ruled states out of what had been a series of Ottoman provinces. The Sunni minority came to rule Iraq; the Alawites came to rule Syria; Lebanon was created to give Maronite Christians a state of their own, but they too were reduced to a minority and then lost power; Jordan’s Hashemite monarchy ruled over a state whose majority today is Palestinian; and in the British colonial entity of Palestine, Jewish immigrants from Europe (who comprised about 45% of the population in 1948) emerged in control.

The U.S. invasion of Iraq ended Sunni minority rule and sent sectarian political shockwaves across the region. Shi’ite majority rule may have been the democratic outcome, but it was never accepted by Iraq’s Sunnis or by their patrons in Saudi Arabia and elsewhere. Sunni-Shi’ite tensions have simmered across the region, flaring up in Lebanon and Bahrain — but Syria could prove to be a game changer.

There are signs that Lebanon’s fragile peace may not survive the fall of Assad, with Saudi-backed Sunni groups tempted to take the opportunity of Hizballah being weakened by the loss of its Syrian patron and arms supplier to break the Shi’ite movement’s political and military dominance. Similarly, the defeated Sunnis of Iraq will take courage from the success of their kin across a border straddled by their tribal and clan networks to push back against the Iran-backed government of Prime Minister Nouri al-Maliki.

Jordan’s pro-Western monarchy is politically weak, and the same bleak economic outlook that drove many of Syria’s rural Sunnis to rebellion prevails in much of the Jordanian hinterland. The triumph of an armed Sunni rebellion in Syria is likely to spur Jordan’s Sunni Islamist opposition — both its more moderate parliamentary arm and its more radical extremist element — to press their case, possibly fueled by an influx of refugees from Syria.

Even Israel has little reason to enthuse about Assad’s fall: his regime postured resistance and empowered Hizballah, but Israel’s border with Syria had been stable for near on four decades under the Assads. Posturing resistance to Israel, in fact, was in part an ideological device through which an Alawite-dominated regime sought to legitimize itself in the eyes of the Sunni majority. Today, however, residents of Syria’s massive Palestinian refugee camps appear to have thrown in their lot with the rebellion, and Hamas broke with Assad and left town last year. Even if concerns about chemical weapons or jihadists on the Golan fail to materialize, Israel could find itself living alongside a new, Sunni-led Syrian polity that, if anything, could be even more insistent than Assad had been on recovering the Golan, occupied by Israel since the 1967 war — and which Israel has no inclination to give up.

When Assad falls, those straight lines on the maps drawn in the foreign offices of France and Britain in the 1920s will start to look even fuzzier than they already are. What happens in Syria is unlikely to stay in Syria.

Tony Karon is a senior editor at TIME, where he has covered international conflicts in the Middle East, Asia, and the Balkans since 1997. A native of South Africa, he now resides with his family in Brooklyn, New York.